The invention relates to an electromechanical brake power assist unit of a vehicle braking system having a master brake cylinder with a piston rod, which can be displaced by way of a brake pedal and, in the process, builds up or reduces brake pressure. The piston rod can also be displaced by way of an electric motor, in which case this single electric motor is activated as a function of the pedal path or the pivot angle of the brake pedal pivotally suspended about an axis of rotation such that the effort to be applied by the driver of the vehicle to the brake pedal for the building-up of brake pressure is less than the effort required without the assistance by the electric motor for the corresponding displacement of the piston rod.
With respect to the state of the art, in addition to German Patent document DE 10 2008 057 576 A1, which describes principles by means of an electro-hydraulic brake power assist unit, and in addition to German Patent document DE 10 2005 036 922 A1, in which a brake pedal mechanism with a changeable transmission ratio is shown, reference is particularly made to German Patent document DE 10 2009 031 918 A1. This last-mentioned document shows an example of the customary principle of electromechanical brake power assist units, according to which the usually single electric motor applying an assisting power acts directly upon the piston rod of the master brake cylinder.
Brake power assist units in vehicle passenger cars even today are still designed mostly as vacuum brake power assist units and use a vacuum for generating the assisting force, which, in case of earlier vehicle drive assemblies in the form of quantity-controlled internal-combustion engines, was available as a quasi “waste product”. In the case of today's modern vehicle drive assemblies, this energy source is no longer or only insufficiently available. An additional vacuum pump is therefore provided in order to be able to continue to use the conventional vacuum brake power assist units. This requires relatively high expenditures; in addition, the relatively high space requirement of a vacuum brake power assist unit is a disadvantage. As an alternative, a hydraulic or an electromechanical brake power assist unit can be used.
The demands made on such, in particular, electro-hydraulic or electromechanical brake power assist units, are very high. The reason is that, on the one hand, these brake power assist units have to meet the highest safety demands and, also in the event of a failure of, for example, an electric component or the like, should still permit a secure braking of the vehicle by the driver. On the other hand, the so-called pedal feel, which the driver of the motor vehicle experiences when actuating his brake pedal, should be as comparable as possible to the pedal feel to which he is accustomed from vacuum brake power assist units, with a harmonious assignment between the pedal force and the pedal path. For a good brake pedal feel, a precise assignment of the pedal force to the pedal path is therefore necessary at least within certain limits. Good characteristic pedal force curves are therefore known from vacuum brake power assist units. An electromechanical assistance should therefore basically also be modulated very precisely simultaneously with the pedal movement. However, in this case, a high-ratio electric motor has a high inertia which limits dynamics. If, for example, the power assistance is slightly slower than the pedal movement, the brake pedal will “harden”; i.e. the assistance is too low for the pedal path and the pedal path—pedal force assignment will then no longer be on the desired characteristic pedal force curve. The same applies when the power assistance takes place too rapidly. In this case, the pedal will “fall through”; i.e. the pedal force is too low for the pedal path. When the dynamics of the actuation of a vehicle braking system are analyzed, it seems to be extremely difficult to achieve the high dynamics of the assisting electric motor that correspond to a vacuum brake power assist unit, particularly also because the brake pedal path has to be very precisely measured and has to be processed by an electronic control unit for this electric motor, before the latter can be activated.
It is an object of the present invention to provide measures for avoiding the above-described problems.
For an electromechanical brake power assist unit of a type described above, a solution is provided in that the piston rod is supported on a pedal lever, which may be formed either by the brake pedal itself or by an intermediate element that can be pivoted about an axis of rotation by way of the brake pedal, and is displaced by a pivotal movement of this pedal lever. The axis of rotation of the pedal lever is displaceable by way of the (single) electric motor essentially in the moving direction of the piston rod.
A principle of an electromechanical brake power assist unit is disclosed that fundamentally differs from the state of the art, for example, from the above-mentioned German Patent document DE 10 2009 031 918 A1, where the electric motor acts directly upon the piston rod of the master brake cylinder. Specifically, according to the present invention, the influence of the electric motor is separate from the piston rod of the master brake cylinder, similar to the also above-mentioned German Patent Document DE 10 2005 036 922 A1, which, however, shows a pneumatic or a conventional electromechanical brake power assist unit having a lever mechanism disposed in front of the brake power assist unit and having a transmission ratio that can be changed by the electric motor.
In the present case, it was recognized that, in principle, no brake power assistance is necessary when the pedal transmission ratio is selected to be sufficiently large. However, in practice, this had the result that the required pedal path can neither be generated by the driver's physiologically possible foot path, nor can be accommodated in a conventional foot well of a motor vehicle, for example, a passenger car. In the present case, the single electric motor is therefore provided for controlling the pedal path to a conventional extent and nevertheless providing a high pedal transmission ratio. According to the invention, the electric motor provides no assisting power in the actual sense for this purpose but quasi-assistance work which, as known, is physically defined as a mathematical product of force and path. By means of the use according to the invention of the electric motor, the effort (or work) to be carried out by the driver for building up a certain brake pressure in the hydraulic vehicle brake system is less than without the use of this electric motor. For this purpose, the electric motor changes the pedal ratio and thereby changes the path, which the driver has to produce at his brake pedal, in such a manner that the work to be performed by the driver for building up the brake pressure will be reduced. In contrast, the force which the driver has to apply for building up this brake pressure is essentially equally high with the use and without the use of this electric motor.
In this sense, the above-mentioned pedal ratio, which is defined as the ratio of the distance between the axis of rotation of the pedal lever and the driver's force introduction point and the distance between the above-mentioned axis of rotation and the supporting point of the piston rod, which is situated on the pedal lever between its axis of rotation and the above-mentioned force introduction point, is preferably selected such that the driver can slightly displace the piston rod by use of forces customary in the case of a conventional effective brake power assistance also without an activation of the electric motor. Simultaneously, the pedal path, which is unusually large because of such a pedal ratio without an activation of the electric motor, will then be reduced to a normal extent by the suitable electric-motor-driven displacement of the axis rotation of the pedal lever, i.e. is controlled such that a desired usual pedal path is obtained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.